Experiments designed to clarify the physiological functions of intracellular bacterial enzymes catalyzing hydrolysis of CO-NH bonds between residues of common protein amino acids in di-, tri-, and oligo-peptides are in progress. Several such amino- and di-peptidases are being purified from Escherichia coli strain K-12 and a peptidase-deficient mutant and their substrate specificity and biochemical characteristics determined. Other peptidase mutants are being sought. When available, the mutants will be compared with the parent strain with respect to (a) growth cycles, (b) resistance to bacteriostatic leucine-containing peptides, (c) ability rapidly to degrade proteins in vivo to free amino acids, and (d) rates of protein synthesis from exogenous amino acids in vivo and in vitro.